The mt7530_{r,w}32 operation over MDIO uses 3 mdiobus operations and
does not hold a lock, which causes a race condition when multiple
threads try to access a register, they may get unexpected results.
To avoid this, handle the MDIO lock manually, and use the unlocked
__mdiobus_{read,write} in the critical section.
This fixes the "Ghost VLAN" artifact[1] in MT7530/7621 when the VLAN
operation and the swconfig LED link status poll race between each other.
[1] https://forum.openwrt.org/t/mysterious-vlan-ids-on-mt7621-device/64495
Signed-off-by: DENG Qingfang <dqfext@gmail.com>
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